Silicon-on-insulator (SOI) substrate

ABSTRACT

There is provided a method of fabricating a silicon-on-insulator substrate, including the steps of (a) forming a silicon substrate at a surface thereof with an oxygen-containing region containing oxygen at such a concentration that oxygen is not precipitated in the oxygen-containing region in later mentioned heat treatment, (b) forming a silicon oxide film at a surface of the silicon substrate, (c) implanting hydrogen ions into the silicon substrate through the silicon oxide film, (d) overlapping the silicon substrate and a support substrate each other so that the silicon oxide film makes contact with the support substrate, and (e) applying heat treatment to the thus overlapped silicon substrate and support substrate to thereby separate the silicon substrate into two pieces at a region into which the hydrogen ions have been implanted, one of the two pieces remaining on the silicon oxide film as a silicon-on-insulator active layer. The support substrate, the silicon oxide film located on the support substrate, and the silicon-on-insulator active layer formed on the silicon oxide film defines a silicon on-insulator structure. The method makes it possible to significantly reduce crystal defect density in the silicon-on-insulator active layer, which ensures that a substrate made in accordance with the method can be used for fabricating electronic devices thereon.

This is a divisional of Application No. 09/292,948 filed Apr. 16, 1999now U.S. Pat. No. 6,211,041, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of fabricating a silicon-on-insulatorsubstrate including an insulator and a silicon active layer formed onthe insulator, and more particularly to such a method including ahydrogen ion separation process. The invention also relates to asilicon-on-insulator substrate suitable for a hydrogen ion separationprocess.

2. Description of the Related Art

A silicon-on-insulator (hereinafter, referred to as “SOI”) structureincluding a silicon active layer formed on an insulator is consideredpromising as a substrate to be used for next generation LSI. There havebeen suggested various methods of fabricating an SOI substrate.

One of such various methods is a method having the steps of forming anoxide film on a surface of a silicon substrate, implanting hydrogen ionsinto the silicon substrate, overlapping the silicon substrate to asupport substrate, and applying heat treatment to the thus overlappedsilicon and support substrates to thereby separate the silicon substrateinto two pieces at a region to which hydrogen ions have been implanted(hereinafter, this method is referred to as “hydrogen ion separationprocess”).

Hereinbelow is explained the above-mentioned hydrogen ion separationprocess with reference to FIGS. 1A to 1F.

First, as illustrated in FIG. 1A, there are prepared a silicon substrate1 and a support substrate 2. A silicon wafer having (100) plane or aplane slightly inclined relative to (100) plane as a principal plane isusually selected as the silicon substrate 1. The same silicon wafer asjust mentioned is usually also selected as the support substrate 2.

Then, as illustrated in FIG. 1B, a silicon dioxide film 3 is formed at asurface of the silicon substrate 1. This silicon dioxide film 3 willmake an insulating film in an SOI structure. Hence, the silicon dioxidefilm 3 is designed to have a thickness equal to a thickness of a buriedoxide film required for fabrication of a device on an SOI substrate.

Then, as illustrated in FIG. 1C, hydrogen is ion-implanted into thesilicon substrate 1 through the silicon dioxide film 3. The thusion-implanted hydrogen 4 stays in the silicon substrate 1 at a certaindepth. When the silicon substrate 1 is subject to heat treatment in alater step, the silicon substrate 1 is separated at that depth into twopieces. One of the two pieces to which the silicon dioxide film 3belongs makes an SOI active layer in an SOI structure. Hence, in thisstep of ion-implanting hydrogen into the silicon substrate 1,acceleration energy is controlled for the SOI active layer to have adesired thickness. The silicon substrate 1 is usually implanted at about30-200 KeV with doses of 1×10¹⁶-3×10¹⁷H⁺ cm⁻². The implanted hydrogenions break bondings between silicon atoms in silicon crystal, andterminate non-bonded hands of silicon atoms.

Then, as illustrated in FIG. 1D, the silicon substrate 1 is laid on topof the support substrate 2 so that surfaces of them make direct contactwith each other. Thereafter, the thus overlapped silicon substrate 1 andsupport substrate 2 are subject to heat treatment.

The heat treatment has two stages.

In a first stage, heat treatment to be carried out at a relatively lowtemperature in the range of 300 to 800 degrees centigrade is applied tothe overlapped silicon substrate 1 and support substrate 2. By carryingout the first stage, the silicon substrate 1 and the support substrate 2make close contact with each other, and at the same time, the siliconsubstrate 1 is separated into two pieces at the depth at which hydrogen4 have been ion-implanted, as illustrated in FIG. 1E.

Hydrogen 4 having been ion-implanted into the silicon substrate 1 in thestep illustrated in FIG. 1C is agglomerated at (111) plane or at (100)plane which is parallel to a surface of the silicon substrate 1, as atemperature raises in the first stage of the heat treatment, to therebyform cavities in the silicon substrate 1. If the support substrate 2 isnot laid on the silicon substrate 1, a surface layer of the siliconsubstrate 1 would be peeled off by pressure of hydrogen gas generated inthe first stage heat treatment carried out at 300-800 degreescentigrade.

However, in accordance with the hydrogen ion separation process, sincethe support substrate 2 makes close contact with the silicon substrate 1with the silicon dioxide film 3 being sandwiched therebetween, thesilicon substrate 1 is separated into two pieces one of which remainsnon-separated from the silicon dioxide film 3 and the support substrate2. One of the two pieces, which remains on the silicon dioxide film 3,acts as an SOI active layer 5. Thus, there is formed an SOI structureincluding the support substrate 2, the silicon dioxide film 3 located onthe support substrate 2, and the SOI active layer 5 formed on thesilicon dioxide film 3. As mentioned above, the SOI active layer 5 isone of the two pieces of the silicon substrate 1.

The separation of the silicon substrate 1 into two pieces is consideredpartially because of force of deformation caused due to a difference ina thermal expansion coefficient between the support substrate 2 and thesilicon dioxide film 3.

Then, in a second stage of the heat treatment, the SOI structureincluding the SOI active layer 5, the silicon dioxide film 3, and thesupport substrate 2 is subject to heat treatment at a relatively hightemperature, specifically, at 1000 degrees centigrade or greater. Thus,as illustrated in FIG. 1F, there is completed an SOI substrate.

The second stage heat treatment is carried out for the purpose ofenhancing bonding force between the support substrate 2 and the silicondioxide film 3, because it would be impossible to ensure sufficientbonding force therebetween only by the first stage heat treatment.

In the specification, a silicon wafer is distinctive from a siliconsubstrate. Specifically, the term “silicon wafer” is used as a genericname for indicating a wafer manufactured by CZ process, for instance,whereas the term “silicon substrate” is used to indicate a substrate onwhich an active layer is to be formed in fabrication of an SOIsubstrate.

When fabrication of an SOI substrate by hydrogen ion separation processis repeated, the other of the two pieces of the silicon substrate 1,removed away in the above-mentioned first stage heat treatment, may bere-used as the silicon substrate 1 or as the support substrate 2 in nextfabrication of an SOI substrate.

For instance, Japanese Unexamined Patent Publications Nos. 2-46770 and9-22993 have suggested fabrication of an SOI substrate by such ahydrogen ion separation process as mentioned above.

Apart from those Publications, fabrication of an SOI substrate by ahydrogen ion separation process has been reported in (a) C. Maleville etal., Silicon-on-Insulator and Devices VII, pp. 34, Electrochem. Soc.,Pennington, 1996, and (b) Abe et al., Applied Physics, Vol. 66, No. 11,pp. 1220, 1997.

The hydrogen ion separation process for fabrication of an SOI substrate,having been explained so far, has many advantages as follows, forinstance, in comparison with other processes for fabrication of an SOIsubstrate.

First, it is possible to control a thickness of an SOI active layer,since a thickness of an SOI active layer is dependent on a rangedistance of ion-implanted hydrogen. The hydrogen ion separation processis suitable in particular for fabrication of a super-thin film SOIsubstrate.

Second, it is possible to uniformize a thickness of an SOI active layer,and an SOI substrate can readily have a large diameter.

Third, it is possible to reduce fabrication cost, because the hydrogenion separation process is comprised of steps of ion-implantation andheat treatment both of which are compatible with an ordinary LSIfabrication process.

Fourth, great designability is ensured for thicknesses of an SOI activelayer and a buried oxide film.

Fifth, an efficiency for using a wafer is higher than that of other SOIsubstrate fabrication processes. Specifically, an SOI substratefabrication process such as a process including the steps of laying afirst substrate on a second substrate, and polishing the first or secondsubstrate to thereby make a thin film requires preparation of two wafersfor fabrication of an SOI substrate. On the other hand, the hydrogen ionseparation process requires preparation of only one wafer forfabrication of an SOI substrate by using a silicon wafer as a supportsubstrate, and re-using a removed piece of a silicon substrate as asilicon or support substrate in next fabrication of an SOI substrate.

However, the hydrogen ion separation process is accompanied with aproblem that a resultant SOI active layer includes a lot of crystaldefects therein. An SOI active layer made in accordance with theconventional hydrogen ion separation process usually includes crystaldefects by the number of about 1×10³ to 1×10⁴/cm². Such a lot of crystaldefects exert harmful influence on characteristics of a device to beformed on an SOI active layer including the crystal defects.

From the standpoint of practical use, it is absolutely necessary toreduce a crystal defect density in an SOI active layer down to about1×10/cm² or smaller in order to use an SOI substrate in next generationLSI.

However, not only a method of reducing crystal defects, but also thereason why crystal defects are generated have not been found so far.

SUMMARY OF THE INVENTION

In view of the above-mentioned problem, it is an object of the presentinvention to make it possible to reduce a crystal defect density in anSOI active layer in fabrication of an SOI substrate by means of ahydrogen ion separation process.

In one aspect of the present invention, there is provided a method offabricating a silicon-on-insulator substrate, including the steps of (a)forming a silicon substrate at a surface thereof with anoxygen-containing region containing oxygen at such a concentration thatoxygen is not precipitated in the oxygen-containing region in latermentioned heat treatment, (b) forming a silicon oxide film at a surfaceof the silicon substrate, (c) implanting hydrogen ions into the siliconsubstrate through the silicon oxide film, (d) overlapping the siliconsubstrate and a support substrate each other so that the silicon oxidefilm makes contact with the support substrate, and (e) applying heattreatment to the thus overlapped silicon substrate and support substrateto thereby separate the silicon substrate into two pieces at a regioninto which the hydrogen ions have been implanted, one of the two piecesremaining on the silicon oxide film as a silicon-on-insulator activelayer, the support substrate, the silicon oxide film located on thesupport substrate, and the silicon-on-insulator active layer formed onthe silicon oxide film defining a silicon-on-insulator structure.

It is preferable that the heat treatment in the step (e) is comprised offirst heat treatment to be carried out at a temperature in the range of300 to 800 degrees centigrade both inclusive, and second heat treatmentto be carried out at a temperature in the range of 1000 to 1200 degreescentigrade.

It is preferable that the oxygen-containing region is designed tocontain oxygen at a concentration equal to or smaller than 1×10¹⁸/cm³.

In the step (c), ions other than hydrogen ions may be also implantedinto the silicon substrate together with the hydrogen ions through thesilicon oxide film.

It is preferable that the method further includes the step (f) offorming an oxide film at a surface of the support substrate, the siliconsubstrate and the support substrate being overlapped each other in thestep (d) so that the silicon oxide films of the silicon substrate andthe support substrate make contact with each other.

It is preferable that the method further includes the step (g) ofcausing at least one of the silicon substrate and the support substrateto absorb hydroxyl group thereinto prior to carrying out the step (d).

For instance, the step (g) may be comprised of the steps of removing anatural oxide film out of a surface of the substrate(s), and rinsing thesubstrate(s) in super-pure water.

It is preferable that the method further includes the step (h) ofselecting a material of which the support substrate is composed, amongone of silicon, quartz glass, sapphire, SiC, and diamond.

It is preferable that the support substrate and the silicon substrateare designed to have common characteristics, and the method furtherincludes the step of (i) using the other of the two pieces of thesilicon substrate as a support substrate in next fabrication of asilicon-on-insulator substrate.

It is preferable that the method further includes the step of (i) usingthe other of the two pieces of the silicon substrate as a siliconsubstrate in next fabrication of a silicon-on-insulator substrate.

There is further provided a method of fabricating a silicon-on-insulatorsubstrate, including the steps of (a) forming a silicon oxide film at asurface of a silicon substrate containing oxygen at such a concentrationthat oxygen is not precipitated in the silicon substrate in latermentioned heat treatment, (b) implanting hydrogen ions into the siliconsubstrate through the silicon oxide film, (c) overlapping the siliconsubstrate and a support substrate each other so that the silicon oxidefilm makes contact with the support substrate, and (d) applying heattreatment to the thus overlapped silicon substrate and support substrateto thereby separate the silicon substrate into two pieces at a regioninto which the hydrogen ions have been implanted, one of the two piecesremaining on the silicon oxide film as a silicon-on-insulator activelayer, the support substrate, the silicon oxide film located on thesupport substrate, and the silicon-on-insulator active layer formed onthe silicon oxide film defining a silicon-on-insulator structure.

It is preferable that the method further includes the step of making thesilicon substrate by FZ process or MCZ process.

There is still further provided a method of fabricating asilicon-on-insulator substrate, including the steps of (a) forming asilicon substrate at a surface thereof with an oxygen-containing regioncontaining oxygen at a lower concentration than a concentration of otherregions of the silicon substrate so that oxygen is not precipitated inthe oxygen-containing region in later mentioned heat treatment, (b)forming a silicon oxide film at a surface of the silicon substrate, (c)implanting hydrogen ions into the silicon substrate through the siliconoxide film, (d) overlapping the silicon substrate and a supportsubstrate each other so that the silicon oxide film makes contact withthe support substrate, and (e) applying heat treatment to the thusoverlapped silicon substrate and support substrate to thereby separatethe silicon substrate into two pieces at a region into which thehydrogen ions have been implanted, one of the two pieces remaining onthe silicon oxide film as a silicon-on-insulator active layer, thesupport substrate, the silicon oxide film located on the supportsubstrate, and the silicon-on-insulator active layer formed on thesilicon oxide film defining a silicon-on-insulator structure.

It is preferable that the step (a) is carried out by applying heattreatment to the silicon substrate at 1000 degrees centigrade or greaterin atmosphere containing oxygen at 1% or smaller, in which case, it ispreferable that the heat treatment is carried out at 1300 degreescentigrade or smaller.

In another aspect of the present invention, there is provided asilicon-on-insulator substrate including (a) a substrate, (b) aninsulating film formed on the substrate, and (c) a silicon layercontaining oxygen at a concentration equal to or smaller than1×10¹⁸/cm³.

It is preferable that the substrate contains hydroxyl group therein.

The substrate may be composed of one of silicon, quartz glass, sapphire,SiC, and diamond.

Hereinbelow is explained the principle of the present invention by whichit is possible to fabricate an SOI substrate having crystal defects bythe smaller number than crystal defects of an SOI substrate fabricatedin accordance with a conventional hydrogen ion separation process.

The inventor inspected the reason why crystal defects are generated, andfound out the following.

First, it was found out that a silicon substrate used in a conventionalSOI substrate contained oxygen in a considerable amount, specifically,at a concentration of 1×10¹⁸/cm³ or greater. In fabrication of an SOIsubstrate from such a silicon substrate by a hydrogen ion separationprocess, two stages heat treatment to be carried out after overlapping asilicon substrate and a support substrate each other generatesprecipitation cores of oxygen contained in a silicon substrate, andfacilitates precipitation of oxygen. As a result, oxygen originallycontained in a silicon substrate is precipitated, and the thusprecipitated oxygen causes deformation in a silicon substrate, which inturn causes dislocation and/or rod-shaped crystal defects. Inconclusion, crystal defects in an SOI active layer fabricated by ahydrogen ion separation process are caused by oxygen originallycontained in a silicon substrate.

It was further found out that if an SOI substrate was made from asilicon substrate containing oxygen at a relatively low concentration,even after the above-mentioned two stage heat treatment was applied,oxygen was suppressed from being precipitated, and as a result, it waspossible to prevent generation of crystal defects in an SOI activelayer.

A crystal defect density of an SOI active layer is not in proportion toan oxygen concentration of a silicon substrate. It was found out, thatif a silicon substrate contained oxygen at a critical concentration orsmaller, crystal defects were no longer generated. It was also found outthat such a critical oxygen concentration was dependent on conditions ofheat treatment to be carried out in an SOI substrate fabricationprocess, and was about 1×10¹⁸/cm³ in ordinary conditions of heattreatment.

The above-mentioned phenomenon is considered that a silicon substratehas to contain oxygen at a critical concentration or greater in orderthat oxygen precipitation cores are generated during heat treatment, andthat such a critical oxygen concentration is equal to about 1×10¹⁸/cm³.

Based on the above-mentioned discovery of the inventor, it is understoodthat it would be possible to prevent generation of crystal defects andthereby fabricate a qualified SOI substrate having less crystal defectsby using a silicon substrate having a surface layer which will make anSOI active layer and which contains oxygen at a concentration of about1×10¹⁸/cm³ or smaller.

In the present invention, there may be used a silicon substratecontaining oxygen at a relatively low concentration. As an alternative,there may be used a silicon substrate containing oxygen at a relativelyhigh concentration, after the silicon substrate is treated to reduce anoxygen concentration at a surface thereof For instance, a siliconsubstrate containing oxygen at a relatively low concentration,fabricated by FZ process or MCZ process, may be used without anypre-treatment. A silicon substrate containing oxygen at a relativelyhigh concentration, fabricated by CZ process, may be used after anoxygen concentration at a surface thereof is reduced, for instance, byapplying heat treatment in argon atmosphere thereto.

In order to strengthen a bonding force between a silicon oxide film tobe formed on a surface of a silicon substrate, and a support substrate,a silicon substrate and/or a support substrate may be caused to absorbhydroxyl group (OH group) thereinto before overlapping them each other.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are cross-sectional views of an SOI substrate,illustrating respective steps of a method of fabricating an SOIsubstrate by a hydrogen ion separation process, in accordance with thefirst embodiment of the present invention.

FIGS. 2A to 2F are cross-sectional views of an SOI substrate,illustrating respective steps of a method of fabricating an SOIsubstrate by a hydrogen ion separation process, in accordance with thesecond embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[First Embodiment]

In the first embodiment, there is used a silicon substrate containingoxygen at a relatively low concentration in place of a silicon substratecontaining oxygen at a relatively high concentration which has beenconventionally used. As explained hereinbelow, it is possible tofabricate an SOI substrate having a reduced crystal defect density inaccordance with the first embodiment having the same steps as those of aconventional hydrogen ion separation process except that a siliconsubstrate containing oxygen at a relatively low concentration is used.

A method of fabricating an SOI substrate in accordance with the firstembodiment is explained hereinbelow with reference to FIGS. 1A to 1F.

First, as illustrated in FIG. 1A, there are prepared a silicon substrate1 and a support substrate 2. As the silicon substrate 1, there is used asilicon wafer containing oxygen at a concentration of 1×10¹⁸/cm³ orsmaller, fabricated by FZ or MCZ process, for instance. The siliconwafer 1 may have any plane azimuth. For instance, the silicon wafer 1may have (100) plane, (111) plane or plane slightly inclined relative to(100) or (111) plane.

There may be used a silicon wafer as the support substrate 2. As analternative, the support substrate 2 may be composed of glass which canwithstand heat treatment to be later carried out at 1000 degreescentigrade or greater, such as quartz glass, sapphire, SiC, or diamond.The support substrate 2 may be formed at a surface thereof with asilicon dioxide film in advance.

Then, as illustrated in FIG. 1B, a silicon dioxide film 3 is formed at asurface of the silicon substrate 1. This silicon dioxide film 3 willmake an insulating film in an SOI structure. Hence, the silicon dioxidefilm 3 is designed to have a thickness equal to a thickness of a buriedoxide film required for fabrication of a device on an SOI substrate.

The silicon dioxide film 3 is preferably formed by thermal oxidation inO₂ or O₂—H₂ atmosphere, because such thermal oxidation provides supercharacteristics as an oxide film to the silicon dioxide film 3. When thesilicon dioxide film 3 is formed by thermal oxidation, not only asurface of the silicon substrate 1 but also all outer surfaces of thesilicon substrate 1 are covered with the silicon dioxide film 3, whichdoes not exert any harmful influence on later steps.

Then, as illustrated in FIG. 1C, hydrogen is ion-implanted into thesilicon substrate 1 through the silicon dioxide film 3. The thusion-implanted hydrogen 4 stays in the silicon substrate 1 at a certaindepth. When the silicon substrate 1 is subject to heat treatment in alater step, the silicon substrate 1 is separated at that depth into twopieces. One of the two pieces to which the silicon dioxide film 3belongs makes an SOI active layer in an SOI structure. Hence, in thisstep of ion-implanting hydrogen into the silicon substrate 1,acceleration energy is controlled so that the SOI active layer has adesired thickness. In the instant embodiment, the silicon substrate 1 isimplanted at about 30-200 KeV with doses of 1×10¹⁶-3×10¹⁷ H⁺ cm⁻². Theimplanted hydrogen ions break bondings between silicon atoms in siliconcrystal, and terminate non-bonded hands of silicon atoms.

Other ions different from hydrogen ions, such as helium ions, may beimplanted into the silicon substrate 1 together with hydrogen ions. Suchion-implantation of hydrogen ions and other ions facilitates separationof the silicon substrate 1 in a later mentioned step.

Then, as illustrated in FIG. 1D, the silicon substrate 1 is laid on topof the support substrate 2 so that surfaces of them make direct contactwith each other. Thereafter, the thus overlapped silicon substrate 1 andsupport substrate 2 are subject to heat treatment, which is carried outin two stages.

In a first stage, heat treatment to be carried out at a relatively lowtemperature in the range of 300 to 800 degrees centigrade is applied tothe overlapped silicon substrate 1 and support substrate 2. When thetemperature is set high, the first stage heat treatment is carried outfor a short period of time, whereas when the temperature is set low, thefirst stage heat treatment is carried out for a long period of time. Forinstance, when the first stage heat treatment is carried out at 800degrees centigrade, the heat treatment lasts for about 10 minutes,whereas when the first stage heat treatment is carried out at 300degrees centigrade, the heat treatment lasts for about 2 hours.

By carrying out the first stage heat treatment, the silicon substrate 1and the support substrate 2 make close contact with each other, and atthe same time, the silicon substrate 1 is separated into two pieces atthe depth at which hydrogen 4 have been ion-implanted, as illustrated inFIG. 1E.

Since the support substrate 2 makes close contact with the siliconsubstrate 1 with the silicon dioxide film 3 being sandwichedtherebetween, the silicon substrate 1 is separated into two pieces oneof which remains non-separated from the silicon dioxide film 3 and thesupport substrate 2. One of the two pieces, which remains on the silicondioxide film 3, makes an SOI active layer 5. Thus, there is formed anSOI structure including the support substrate 2, the silicon dioxidefilm 3 located on the support substrate 2, and the SOI active layer 5formed on the silicon dioxide film 3. As mentioned above, the SOI activelayer 5 is one of the two pieces of the silicon substrate 1.

Then, in a second stage of the heat treatment, an SOI structureincluding the SOI active layer 5, the silicon dioxide film 3, and thesupport substrate 2 is subject to heat treatment at a relatively hightemperature to thereby enhance a bonding force between the supportsubstrate 2 and the silicon dioxide film 3. Thus, as illustrated in FIG.1F, there is completed an SOI substrate.

The second stage heat treatment is carried out at about 1000 to 1200degrees centigrade. Similarly to the first stage heat treatment, whenthe temperature is set high, the second stage heat treatment is carriedout for a short period of time, whereas when the temperature is set low,the second stage heat treatment is carried out for a long period oftime. For instance, when the second stage heat treatment is carried outat 1200 degrees centigrade, the heat treatment lasts for about 30minutes, whereas when the second stage heat treatment is carried out at1000 degrees centigrade, the heat treatment lasts for about 4 hours.

The thus fabricated SOI structure was estimated with respect tocrystallinity of the SOI active layer 5 by a selective etching process,a transmission electron microscope process, an X-ray topography process,and a photoluminescence process. The results were that a crystal defectdensity of the SOI active layer 5 was equal to or smaller than1×10¹⁸/cm³, if the SOI structure was fabricated in the conditionsmentioned in the first embodiment. The results are sufficiently suitablefor fabrication of LSI which is required to be fabricated smaller andsmaller in size. Thus, the SOI structure in accordance with the instantembodiment makes it possible to completely solve the above-mentionedproblem of a conventional hydrogen ion separation process.

Hereinbelow are explained examples of the first embodiment and referenceexamples.

EXAMPLE 1

In the first example, an SOI substrate was fabricated by a hydrogen ionseparation process, using a silicon wafer, as the silicon substrate 1,which was fabricated by FZ process and contained oxygen at aconcentration of 2×10¹⁷/cm³.

In the first example and all of examples and reference examplesmentioned hereinbelow, a silicon wafer used was selected to have (100)plane as a principal plane.

In the first example, there were prepared six different supportsubstrates A to F, as follows.

1. Support Substrate A

The support substrate A has the same structure as the silicon substrate

1. That is, the support substrate A was a silicon wafer fabricated by FZprocess and containing oxygen at a concentration of 2×10¹⁷/cm³.

2. Support Substrate B

The support substrate B was fabricated by applying heat treatment at1000 degrees centigrade in O₂ atmosphere to the support substrate A forthermal oxidation. Accordingly, the support substrate B was formed at asurface thereof with a 0.1 μm-thick oxide film.

3. Support Substrate C

The support substrate C was a silicon wafer fabricated by CZ process andcontaining oxygen at a concentration of 2×10¹⁸/cm³.

4. Support Substrate D

The support substrate D was fabricated by applying heat treatment at1000 degrees centigrade in O₂ atmosphere to the support substrate C forthermal oxidation. Accordingly, the support substrate D was formed at asurface thereof with a 0.1 μm-thick oxide film.

5. Support Substrate E

The support substrate E was composed of quartz glass.

6. Support Substrate F

The support substrate F was composed of quartz glass and formed with ata surface thereof with a 0.1 μm-thick silicon dioxide film by chemicalvapor deposition (CVD).

Then, heat treatment at 1000 degrees centigrade in O₂ atmosphere wasapplied to the silicon substrate 1 to thereby form a 0.1 μm-thicksilicon dioxide film 3 at a surface of the silicon substrate 1, asillustrated in FIG. 1B.

Then, as illustrated in FIG. 1C, hydrogen was ion-implanted into thesilicon substrate 1 through the silicon dioxide film 3. The thusion-implanted hydrogen 4 stayed in the silicon substrate 1 at a certaindepth. The silicon substrate 1 was implanted at 80 KeV with doses of5×10¹⁶H⁺cm⁻².

Then, the silicon substrate 1 having the structure as illustrated inFIG. 1C was laid on top of each one of the support substrates A to F sothat surfaces of them make direct contact with each other, asillustrated in FIG. 1D. When the silicon substrate 1 illustrated in FIG.1C was laid on top of the substrates B, D and F all of which had asilicon dioxide film at a surface, the silicon dioxide film 3 of thesilicon substrate 1 and the silicon dioxide film of the supportsubstrates B, D and F were overlapped each other.

Then, the thus overlapped silicon substrate 1 and support substrate 2were subject to heat treatment at 500 degrees centigrade for 1 hour. Asa result, the silicon substrate 1 was broken at a region at whichhydrogen ions 4 have been implanted, into two pieces as illustrated inFIG. 1E. One of the two pieces of the silicon substrate 1 remains on thesilicon dioxide film 3, and makes an SOI active layer 5.

Then, the thus fabricated structure including the support substrate 2,the silicon dioxide film 3, and the SOI active layer 5 was subject toheat treatment at 1100 degrees centigrade for 2 hours to thereby enhancea bonding force between the support substrate 2 and the silicon dioxidefilm 3.

Thus, there was completed an SOI structure, as illustrated in FIG. 1F.

In such a manner as mentioned above, there were fabricated six differentSOI structures each including the support substrate A, B, C, D, E or F,the silicon dioxide layer 3, and the SOI active layer 5. Those six SOIstructures had an SOI active layer having a crystal defect densitysmaller than 10 cm⁻².

EXAMPLE 2

In the second example, an SOI substrate was fabricated by a hydrogen ionseparation process, using a silicon wafer, as the silicon substrate 1,which was fabricated by MCZ process and contained oxygen at aconcentration of 5×10¹⁷/cm³.

In the second example, there were prepared six different supportsubstrates A to F, as follows.

1. Support Substrate A

The support substrate A has the same structure as the silicon substrate1. That is, the support substrate A was a silicon wafer fabricated byMCZ process and containing oxygen at a concentration of 5×10¹⁷/cm³.

2. Support Substrate B

The support substrate B was fabricated by applying heat treatment at1000 degrees centigrade in O₂ atmosphere to the support substrate A forthermal oxidation. Accordingly, the support substrate B was formed at asurface thereof with a 0.1 μm-thick oxide film.

3-6. Support Substrates C-F

The support substrates C to F were the same as the support substrates Cto F having been used in the first example.

In the same manner as the above-mentioned first example, there werefabricated six different SOI structures each including the supportsubstrate A, B, C, D, E or F, the silicon dioxide layer 3, and the SOIactive layer 5.

Those six SOI structures had an SOI active layer having a crystal defectdensity smaller than 10 cm⁻².

Reference Example

In the reference example, an SOI substrate was fabricated by aconventional hydrogen ion separation process, using a silicon wafer, asthe silicon substrate 1, which was fabricated by CZ process andcontained oxygen at a concentration of 2×10¹⁸/cm³.

In the reference example, there were prepared six different supportsubstrates A to F, which were the same as the support substrates A to Fhaving been used in the first example.

In the same manner as the above-mentioned first example, there werefabricated six different SOI structures each including the supportsubstrate A, B, C, D, E or F, the silicon dioxide layer 3, and the SOIactive layer 5.

The thus fabricated six SOI structures had an SOI active layer having acrystal defect density in the range of 1×10³ to 1×10⁴ cm⁻². The reasonwhy the SOI active layer in the reference example had so high crystaldefect density was that the SOI active layer contained a lot of crystaldefects such as oxygen precipitate, dislocation caused by deformation inthe SOI active layer which was in turn caused by oxygen precipitate, androd-shaped defects. Those crystal defects were not found in the SOIsubstrates made in accordance with the above-mentioned first and secondexamples.

In view of comparison between the first and second examples and thereference example, it is understood that a crystal defect density of anSOI active layer can be decreased by using a silicon substratecontaining oxygen at a relatively low concentration. Since a crystaldefect density is not dependent on a particular support substrate, theremay be selected a support substrate suitable for an SOI substrate to befabricated.

If a silicon substrate containing oxygen at a relatively lowconcentration is used as both the silicon substrate 1 and the supportsubstrate 2, as mentioned in the above-mentioned first and secondexamples wherein the supports substrates A and B are composed of thesame silicon wafer as the silicon substrate 1, an unnecessary piece ofthe silicon substrate 1, removed away in the step illustrated in FIG.1E, may be used as a support substrate in next SOI fabrication. Hence, asilicon wafer is wholly used for fabrication of an SOI substrate withoutany waste.

As an alternative, an unnecessary piece of the silicon substrate 1,removed away in the step illustrated in FIG. 1E, may be used as asilicon substrate in next SOI fabrication, in which case, it is notnecessary for a support substrate to be composed of a silicon wafer.

[Second Embodiment]

In the second embodiment, a silicon wafer containing oxygen at arelatively high concentration is used as a silicon substrate after aregion thereof close to a surface, at which an SOI active layer is to beformed, has been treated so as to reduce an oxygen concentration in theregion. In the above-mentioned first embodiment, a silicon wafercontaining oxygen at a relatively low concentration was used as thesilicon substrate 1. However, an SOI active layer on which a device isto be fabricated occupies merely a part of a surface of a siliconsubstrate. Accordingly, if a region at which an SOI active layer is tobe fabricated is designed to contain oxygen at a relatively lowconcentration, other region of the silicon substrate is allowed tocontain oxygen at a relatively high concentration.

A method of fabricating an SOI substrate in accordance with the secondembodiment is explained hereinbelow with reference to FIGS. 2A to 2F.

In the second embodiment, a silicon substrate 1 is first processed so asto have a region 6 which contains oxygen at a relatively lowconcentration, as illustrated in FIG. 2A.

The silicon substrate 1 can be caused to have the region 6 by applyingheat treatment at about 1000 to 1300 degrees centigrade in atmospherecontaining oxygen at a concentration of about 1% or smaller to thesilicon substrate 1 to thereby form a denuded zone at a region in thevicinity of a surface of the silicon substrate 1. The above-mentionedatmosphere may be established as inert gas atmosphere such as argonatmosphere.

As an alternative, a silicon epitaxial layer may be grown on a surfaceof the silicon substrate 1 in a conventional manner. The thus grownsilicon epitaxial layer defines the region 6 containing oxygen at arelatively low concentration.

The region 6 is formed to have a thickness equal to or greater than athickness of a later formed SOI active layer 5. Herein, a thickness ofthe SOI active layer 5 is equal to a depth of the silicon substrate 1 towhich hydrogen ions are implanted in a later mentioned step illustratedin FIG. 2C.

The region 6 is designed to contain oxygen at a concentration equal toor smaller than 1×10¹⁸/cm³.

After the formation of the region 6 at a surface of the siliconsubstrate 1, the same steps as the steps of the first embodiment arecarried out, as illustrated in FIGS. 2A to 2F.

First, as illustrated in FIG. 2A, there is prepared a support substrate2.

There may be used a silicon wafer as the support substrate 2. As analternative, the support substrate 2 may be composed of quartz glass,sapphire, SiC, or diamond. The support substrate 2 may be formed at asurface thereof with a silicon dioxide film in advance.

Then, as illustrated in FIG. 2B, a silicon dioxide film 3 is formed at asurface of the silicon substrate 1.

Then, as illustrated in FIG. 2C, hydrogen is ion-implanted into thesilicon substrate 1 through the silicon dioxide film 3. The thusion-implanted hydrogen 4 stays in the silicon substrate 1 at a certaindepth.

Then, as illustrated in FIG. 2D, the silicon substrate 1 is laid on topof the support substrate 2 so that surfaces of them make direct contactwith each other. Thereafter, the thus overlapped silicon substrate 1 andsupport substrate 2 are subject to heat treatment, which is carried outin two stages.

In a first stage, heat treatment to be carried out at a relatively lowtemperature in the range of 300 to 800 degrees centigrade is applied tothe overlapped silicon substrate 1 and support substrate 2.

By carrying out the first stage heat treatment, the silicon substrate 1and the support substrate 2 make close contact with each other, and atthe same time, the silicon substrate 1 is separated into two pieces atthe depth at which hydrogen 4 have been ion-implanted, as illustrated inFIG. 2E.

Since the support substrate 2 makes close contact with the siliconsubstrate 1 with the silicon dioxide film 3 being sandwichedtherebetween, the silicon substrate 1 is separated into two pieces, oneof which remains non-separated from the silicon dioxide film 3 and thesupport substrate 2. One of the two pieces, which remains on the silicondioxide film 3, makes an SOI active layer 5. Thus, there is formed anSOI structure including the support substrate 2, the silicon dioxidefilm 3 located on the support substrate 2, and the SOI active layer 5formed on the silicon dioxide film 3.

Then, in a second stage of the heat treatment, an SOI structureincluding the SOI active layer 5, the silicon dioxide film 3, and thesupport substrate 2 is subject to heat treatment at a relatively hightemperature to thereby enhance a bonding force between the supportsubstrate 2 and the silicon dioxide film 3. Thus, as illustrated in FIG.2F, there is completed an SOI substrate.

Similarly to the first embodiment, one of two pieces of the siliconsubstrate 1, removed away in the step illustrated in FIG. 2E, may beused as a silicon substrate or a support substrate in next fabricationof an SOI substrate.

EXAMPLE 3

In the third example, there was used a silicon wafer fabricated by CZprocess and containing oxygen at a concentration of as the siliconsubstrate 1. The silicon substrate 1 was formed at a surface thereofwith the region 6 containing oxygen at a relatively low concentration.Then, an SOI substrate was fabricated by a hydrogen ion separationprocess.

The region 6 was formed by applying heat treatment in argon atmosphereto the silicon substrate 1 to thereby form a denuded zone. The heattreatment was carried out at 1200 degrees centigrade at atmosphericpressure in 100%-argon atmosphere for 5 hours. The thus formed region 6had a depth of 10 μm and contained oxygen at a concentration of8×10¹⁷/cm³.

In the third example, there were prepared four different supportsubstrates A to D, as follows.

1. Support substrate A

The support substrate A was a silicon wafer fabricated by CZ process andcontaining oxygen at a concentration of 2×10¹⁸/cm³.

2. Support Substrate B

The support substrate B was fabricated by applying heat treatment at1000 degrees centigrade in O₂ atmosphere to the support substrate A forthermal oxidation. Accordingly, the support substrate B was formed at asurface thereof with a 0.1 μm-thick oxide film.

3. Support Substrate C

The support substrate C was a substrate composed of quartz glass.

4. Support Substrate D

The support substrate D was a quartz glass substrate, and was formed ata surface thereof with a 0.1 μm-thick silicon dioxide film by CVD.

The SOI substrate in accordance with the third example was fabricated inthe same manner as the SOI substrate in accordance with the firstexample. That is, there were fabricated four different SOI structureseach including the support substrate A, B, C, or D, the silicon dioxidelayer 3, and the SOI active layer 5. Those four SOI structures had anSOI active layer having a crystal defect density smaller than 10 cm⁻².

[Third Embodiment]

In the third embodiment, before the silicon substrate 1 is laid on topof the support substrate 2, the silicon substrate 1 and/or the supportsubstrate 2 are(is) caused to absorb hydroxyl group (OH group) thereintofor enhancing a bonding force therebetween.

In the third embodiment, the silicon substrate 1 is selected among thesilicon substrates 1 having been used in the above-mentioned first,second and third examples. There is used a silicon wafer as the supportsubstrate 2.

The method of fabricating an SOI substrate in accordance with the thirdembodiment has the same steps of the methods of fabricating SOIsubstrates in accordance with the first and second embodiments, but hasan additional step of causing hydroxyl group to be absorbed into thesilicon substrate 1 and/or the support substrate 2 before thosesubstrates 1 and 2 are overlapped each other. The substrates 1 and 2 canbe caused to absorb hydroxyl group thereinto, for instance, by applyinghydrofluoric (HF) acid treatment to the substrates 1 and/or 2 to therebyremove a natural oxidation film at a surface of the substrates 1 and/or2, and then, rinsing the substrates 1 and/or 2 with super-pure water.

By causing the silicon substrate 1 and/or the support substrate 2 toabsorb OH group thereinto, it is possible to enhance a bonding forcebetween the substrates 1 and 2. Thus, there is obtained an SOI substratehaving high practicability due to the advantages provided by theinvention that a crystal defect density in the SOI active layer 5 can bedecreased through the use of the silicon substrate 1 containing oxygenat a relatively low concentration, and that a bonding force between thesilicon substrate 1 and the support substrate 2 can be enhanced, asmentioned above.

EXAMPLE 4

In the fourth example, there were used the silicon substrate 1constituted of a silicon wafer fabricated by FZ process and containingoxygen at a concentration of 2×10¹⁷/cm³, and the support substrate 2constituted of a silicon wafer fabricated by CZ process and containingoxygen at a concentration of 2×10¹⁸/cm³.

In the same way as the above-mentioned first example, the siliconsubstrate 1 was formed at a surface thereof with the silicon dioxidefilm 3, and hydrogen ions 4 were implanted into the silicon substrate 1through the silicon dioxide film 3.

Then, the silicon substrate 1 and the support substrate 2 were treatedin the following conditions A to D.

1. Condition A

Both the silicon substrate 1 and the support substrate 2 were subject to49%-HF treatment to thereby remove a natural oxide film therefrom, andthen, were rinsed with super-pure water to thereby cause OH group to beabsorbed into the substrates 1 and 2 at their surfaces.

2. Condition B

Only the silicon substrate 1 was caused to absorb OH group thereinto inthe same way as the condition A.

3. Condition C

Only the support substrate 2 was caused to absorb OH group thereinto inthe same way as the condition A.

4. Condition D

Neither of the silicon substrate 1 and the support substrate 2 werecaused to absorb OH group thereinto.

After the above-mentioned surface treatment of the silicon and supportsubstrates 1 and 2 in the conditions A to D, each one of the siliconsubstrates 1 was laid on top of an associated one of the supportsubstrates 2. Thereafter, the thus overlapped four silicon and supportsubstrates 1 and 2 were subject to twostage heat treatment in the sameway as the first example 1 to thereby fabricate four SOI substrates.

Those four SOI structures had an SOI active layer having a crystaldefect density smaller than 10 cm⁻².

A bonding force between the support substrate 2 and the silicon dioxidefilm 3 in each of the SOI structures was measured by tensile stressprocess. It was found out that a bonding force was greater in the orderof the conditions A, C, B and D. This result means that a bonding forcecan be increased by causing the silicon substrate 1 and/or the supportsubstrate 2 to absorb OH group thereinto.

Though the support substrate 2 was constituted of a silicon wafer in theinstant embodiment, the support substrate 2 may be composed of quartzglass, sapphire, SiC or diamond. As an alternative, the supportsubstrate 2 composed of any one of those materials may be formed at asurface thereof with a silicon dioxide film.

It should be noted that conditions for fabricating an SOI substrate inaccordance with each of the first to third embodiments might bemodified. For instance, a method of and conditions for forming a silicondioxide film on the silicon and support substrates 1 and 2 may beselected among conventional ones. Conditions for carrying out heattreatment, such as temperature, time and atmosphere, may be also varied.For instance, a washing step may be carried out between theabove-mentioned steps of fabricating an SOI substrate.

While the present invention has been described in connection with thepreferred embodiments and examples, the present invention makes itpossible to significantly reduce a crystal defect density in an SOIactive layer by using a silicon substrate containing oxygen at arelatively low concentration in fabrication of an SOI substrate. Thisensures that a substrate made in accordance with the present inventioncan be used for fabricating electronic devices thereon

Thus, the present invention gives life to the earlier mentionedadvantages of a hydrogen ion separation process, and provides highlyqualified SOI-structured electronic devices at low cost.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

The entire disclosure of Japanese Patent Application No. 10-108202 filedon Apr. 17, 1998 including specification, claims, drawings and summaryis incorporated herein by reference in its entirety.

What is claimed is:
 1. A silicon-on-insulator substrate comprising: (a)a substrate; (b) an insulating film formed on said substrate; (c) asilicon layer formed on said insulating film, said silicon layercontaining oxygen at a concentration equal to or smaller than1×10¹⁸/cm³; and (d) said substrate, said insulating film and saidsilicon layer forming a silicon-on-insulator substrate with asilicon-on-insulator active layer having a crystal defect density of ≦10cm ⁻².
 2. The silicon-on-insulator substrate in claim 1, wherein saidsubstrate contains hydroxyl groups therein.
 3. The silicon-on-insulatorsubstrate in claim 1, wherein said substrate is composed of one ofsilicon, quartz glass, sapphire, SiC, and diamond.
 4. Asilicon-on-insulator substrate including a silicon-on-insulator activelayer having a crystal defect density of 10 cm⁻², saidsilicon-on-insulator substrate being resulted from: (a) forming asilicon substrate thereof with an oxygen-containing region containingoxygen at such a concentration that oxygen is not precipitated in saidoxygen-containing region in later mention ed heat treatment; (b) forminga silicon oxide film at a surface of said silicon substrate; (c)implanting hydrogen ions into said silicon substrate through saidsilicon oxide film; (d) placing said silicon substrate on a surface of asupport substrate so that a top surface of said silicon oxide film makescontact with said surface of said support substrate to form a layeredsubstrate; and (e) applying heat treatment to said layered substrate tothereby separate said silicon substrate in to two pieces at a regioninto which said hydrogen ions have been implanted, one of said twopieces remaining on said silicon oxide film as a silicon-on-insulatoractive layer.
 5. The silicon-on-insulator substrate as set forth inclaim 4, wherein said heat treatment is comprised of first heattreatment to be carried out at a temperature in the range of 300 to 800degrees centigrade both inclusive, and second heat treatment to becarried out at a temperature in the range of 1000 to 1200 degreescentigrade.
 6. The silicon-on-insulator substrate as set forth in claim4, wherein said oxygen-containing region contains oxygen at aconcentration equal to or smaller than 1×10¹⁸/cm³.
 7. Thesilicon-on-insulator substrate as set forth in claim 4, wherein ionsother than hydrogen ions are also implanted into said silicon substratetogether with said hydrogen ions through said silicon oxide film in saidstep (c).
 8. The silicon-on-insulator substrate as set forth in claim 4,wherein said silicon-on-insulator substrate is resulted further from (1)of forming an oxide film at a surface of said support substrate, saidsilicon substrate overlapping said support substrate in said step (d) sothat said silicon oxide film of said silicon substrate and said supportsubstrate make contact with each other.
 9. The silicon-on-insulatorsubstrate as set forth in claim 4, wherein said silicon-on-insulatorsubstrate is resulted further from the step (g) of causing at least oneof said silicon substrate and said support substrate to absorb hydroxylgroups there into prior to carrying out said step (d).
 10. Thesilicon-on-insulator substrate as set forth in claim 9, wherein saidstep (g) is comprised of the steps of removing a natural oxide film outof a surface of said substrate(s), and rinsing said substrate(s) insuper-pure water.
 11. The silicon-on-insulator substrate as set forth inclaim 4, wherein said silicon-on insulator substrate is resulted furtherfrom the step (h) of selecting a material of which said supportsubstrate is composed, among one of silicon, quartz glass, sapphire,SiC, and diamond.
 12. The silicon-on-insulator substrate as set forth inclaim 4, wherein said support substrate and said silicon substrate aredesigned to have common characteristics, and further comprising the stepof (i) using the other of said two pieces of said silicon substrate as asupport substrate in next fabrication of a silicon-on-insulatorsubstrate.
 13. The silicon-on-insulator substrate as set forth in claim4, wherein said silicon on-insulator substrate is resulted further fromthe step of (i) using the other of said two pieces of said siliconsubstrate as a silicon substrate in next fabrication of asilicon-on-insulator substrate.
 14. A silicon-on insulator substrateincluding a silicon-on-insulator active layer having a crystal defectdensity of 10 cm⁻², said silicon-on-insulator being resulted from thesteps of: (a) forming a silicon oxide film at a surface of a siliconsubstrate containing oxygen at such a concentration that oxygen is notprecipitated in said silicon substrate in later mentioned heattreatment; (b) implanting hydrogen ions into said silicon substratethrough said silicon oxide film; (c) placing said silicon substrate on asurface of support substrate so that a top surface of said silicon oxidefilm makes contact with said surface of said support substrate to form alayered substrate; and (d) applying heat treatment to substrate tothereby separate said silicon substrate into two pieces remaining onsaid silicon oxide film as a silicon-on-insulator active layer, saidsupport substrate, said silicon oxide film located on said supportsubstrate, and said silicon-on-insulator active layer formed on saidsilicon oxide film defining a silicon-on-insulator structure.
 15. Thesilicon-on-insulator substrate as set forth in claim 14, wherein saidsilicon-on-insulator substrate is resulted further from the step ofmaking said silicon substrate by FZ process or MCZ process.
 16. Thesilicon-on-insulator substrate as set forth in claim 14, wherein saidheat treatment in said step (d) is comprised of first heat treatment tobe carried out at a temperature in the range of 300 to 800 degreescentigrade both inclusive, and second heat treatment to be carried outat a temperature in the range of 1000 to 1200 degrees centigrade. 17.The silicon-on-insulator substrate as set forth in claim 4, wherein saidsilicon substrate contains oxygen at a concentration equal to or smallerthan 1×10¹⁸/cm³.
 18. The silicon-on-insulator substrate as set forth inclaim 14, wherein ions other than hydrogen ions are also implanted intosaid silicon substrate together with said hydrogen ions through saidsilicon oxide film in said step (b).
 19. The silicon-on-insulatorsubstrate as set forth in claim 14, wherein said silicon-on-insulatorsubstrate is resulted further from the step (e) of forming an oxide filmat a surface of said support substrate, aid silicon substrateoverlapping said support substrate in said step (c) so that said siliconoxide films of said silicon substrate and said support substrate makecontact with each other.
 20. The silicon-on insulator substrate as setforth in claim 14, wherein said silicon-on-insulator substrate isresulted further from the step (f) of causing at least one said siliconsubstrate and said support substrate of absorb hydroxyl groups thereuntoprior to carrying out said step (c).
 21. The silicon-on-insulatorsubstrate as set forth in claim 20, wherein said step (f) is comprisedof the steps of removing a natural oxide film out of a surface of saidsubstrate(s) and rinsing said substrate(s) in super-pure water.
 22. Thesilicon-on-insulator substrate as set forth in claim 14, wherein saidsilicon-on-insulator substrate is resulted further from the step (g) ofselecting a material of which said support substrate is composed, amongone of silicon, quartz glass, sapphire, SiC, and diamond.
 23. Thesilicon-on-insulator substrate as set forth in claim 14, wherein saidsupport substrate and said silicon substrate are designed to have commoncharacteristics, and further comprising the step of (h) using the otherof said two pieces of said silicon substrate as a support substrate innext fabrication of a silicon-on-insulator substrate.
 24. Thesilicon-on-insulator substrate as set forth in claim 14, wherein saidsilicon-on-insulator substrate is resulted further from the step of (h)using the other of said two pieces of said silicon substrate as asilicon substrate in next fabrication of a silicon-on-insulatorsubstrate.
 25. A silicon-on-insulator substrate including asilicon-on-insulator active layer having a crystal defect density of 10cm⁻², said silicon-on-insulator substrate being resulted from the stepsof: (a) forming a silicon substrate at a surface thereof with anoxygen-containing region containing oxygen at a lower concentration thana concentration of other regions of said silicon substrate so thatoxygen is not precipitated in said oxygen-containing region in latermentioned heat treatment; (b) forming a silicon oxide film at a surfaceof said silicon substrate; (c) implanting hydrogen ions into saidsilicon substrate through said silicon oxide film; (d) placing saidsilicon substrate on a surface of a support substrate so that a topsurface of said silicon oxide film makes contact with said surface ofsaid support substrate to form a layered substrate; and (e) applyingheat treatment to said layered substrate to thereby separate saidsilicon substrate into two pieces remaining on said silicon oxide filmas a silicon-on-insulator active layer; said support substrate, saidsilicon oxide film located on said support substrate, and saidsilicon-on-insulator active layer formed on said silicon oxide filmdefining a silicon-on-insulator structure.
 26. The silicon-on-insulatorsubstrate as set forth in claim 25, wherein said step (a) is carried outby applying heat treatment to said silicon substrate at 1000 degreescentigrade or greater in atmosphere containing oxygen at 1% or smaller.27. The silicon-on-insulator substrate as set forth in claim 26, whereinsaid heat treatment is carried out at 1300 degrees centigrade orsmaller.
 28. The silicon-on-insulator substrate as set forth in claim25, wherein said heat treatment is said step (e) is comprised of firstheat treatment to be carried out at a temperature in the range of 300 to800 degrees centigrade both inclusive, and second heat treatment to becarried out at a temperature in the range of 1000 to 1200 degreescentigrade.
 29. The silicon-on-insulator substrate as set forth in claim25, wherein said oxygen containing region contains oxygen at aconcentration equal to or smaller than 1×10¹⁸/cm³.
 30. Thesilicon-on-insulator substrate as set forth in claim 25, wherein ionsother than hydrogen ions are also implanted into said silicon substratetogether with said hydrogen ions through said silicon oxide film in saidstep ((c).
 31. The silicon-on-insulator substrate as set forth in claim25, wherein said silicon-on-insulator substrate is resulted further fromthe step (l) of forming an oxide film at a surface of said supportsubstrate, said silicon substrate overlapping said support substrate insaid step (d) so that said silicon oxide films of said silicon substrateand said support make contact with each other.
 32. Thesilicon-on-insulator substrate as set forth in claim 25, wherein saidsilicon-on-insulator is resulted further from the step (g) of causing atleast one of said silicon substrate and said support substrate to absorbhydroxyl groups there into prior to carrying out said step (d).
 33. Thesilicon-on-insulator substrate as set forth in claim 32, wherein saidstep (g) is comprised of the steps of removing on a natural oxide filmout of a surface of said substrate(s) and rinsing said substrate(s) insuper-pure water.
 34. The silicon-on-insulator substrate as set forth inclaim 25, wherein said silicon-on-insulator substrate is resultedfurther from the step (h) of selecting a material of which said supportsubstrate is composed among one silicon quartz glass, sapphire, SiC anddiamond.
 35. The silicon-on-insulator substrate as set forth in claim25, wherein said support substrate and said silicon substrate asdesigned to have common characteristics and further comprising the stepof (i) using the other of said two piece of said silicon substrate as asupport substrate in next fabrication of a silicon-on-insulatorsubstrate.
 36. The silicon-on-insulator as set forth in claim 25,wherein said silicon-on-insulator is resulted further from step of (i)using the other of said two pieces of said silicon substrate as asilicon substrate in next fabrication of silicon-on-insulator substrate.